This substack article came to me by way of a commenter asking if I was interested in it. Of course, I am. It is a part of healthcare and covers a topic I believe is important to all of us. Stopping the transmission of Covid.
On Monday, a Florida judge voided the U.S. mandate for public transit, which includes planes, trains, and buses. Several airlines immediately announced they dropped the mask requirement. In true pandemic fashion, an intense debate about masks ensued.
In a peer review by the American Bar Association pre-appointment, the rating for the deciding judge was “unqualified” to be a district judge. The ABA took issue with the short time she practiced law and her lack of meaningful trial experience.
Ok, so now we have a court making a decision impacting the health of US citizenry. “The CDC is “still” recommending wearing a mask on public transit.” The mask requirement for travelers was the target of intense lobbying by airlines seeking to kill it. Carriers arguing the use of effective air filters on modern planes makes transmission of the virus during a flight unlikely. But is this true?
Airline Story and Filtration
Airline CEOs are claiming the removal of “99.97 of airborne pathogens by filters.” Therefore, masking is serving no purpose.” The first claim may be true, the second claim is not true. Why?
Here is where I turn to an expert the same as I might ask Joel Eissenberg a question on Covid vaccines. Epidemiologist Katelyn Jetelina has much to say on the topic.
Airplanes have fantastic systems with an estimated 10-20 air changes per hour while a hospital has 6 air changes per hour. A DOD report found plane ventilation and filtration systems were reducing the risk of airborne SARS-CoV-2 exposure by 99%. Due to air changes per hour, transmission occurs less frequently even with numerous people in close quarters taking in shared air. After reviewing 18 peer-reviewed studies (public health reports) of flights that were published between January 24, 2020 to September 21, 2020, it was concluded the “transmission of SARS-CoV-2 can still occur in aircrafts. However, it is a relatively rare event.”
Like any mitigation layer, ventilation/filtration isn’t perfect in stopping transmission. For example, you need to get to the airplane to be safer. Airport spaces such as crowded boarding areas, do not have great ventilation. Also, the filtrations systems on planes are not turned on during the boarding process.
SARS-CoV-2 is spread through aerosols and droplets. Filtration is great for floating aerosols, which can suspend in the air for hours. If the air is not filtering first, a passenger risks inhaling the SARS-CoV-2 aerosols in the air before filtering. Filtration is also not effective for larger droplets
Proximity to the index case (i.e., person originally infected before boarding) matters
An extensive study traced 217 passengers and crew from a 10-hour flight from London → Vietnam in March 2020. Masks were not mandatory nor widely used. The index (infected) case was in business class and symptomatic with fever and cough. Sixteen cases were acquired in-flight (i.e., secondary cases). Twelve were in business class, equating to a 75% attack rate in business class. Two cases were in economy class and another case was a staff member.
Assessing another flight study from Israel → Germany in March 2020 with no masks. Secondary cases were two rows away from the index case.
Proximity is important and consistent with other viral outbreaks on planes. In a review of 14 studies, researchers found an overall influenza attack rate of 7.5%. Forty-two percent of the cases were sitting d within two rows of the index case. Similar findings were documented with SARS on a flight. Thirty-four percent were within 3 rows of the index case. As opposed to an 11% attack rate among persons seated elsewhere. There are many examples of secondary cases not in close proximity.
People move around a lot on planes
Nobody sits forever. Hence greater transmission or exposure. Pre-pandemic, one group traveled on 10 intercontinental flights to assess the behaviors and movements of people on planes and the impact on viral transmission. Of the 1,296 passengers observed, 38% left their seat once, 13% left twice, and 11% left more than two times. Eighty-four percent of passengers had a close contact with an individual seated beyond a 1-meter radius from them. People with the most contacts were sitting in the aisle compared to the window.
The assumption would be people in aisle seats have a higher risk of infection. A SARS-CoV-2 study found the opposite. The attack rate is higher for passengers in window seats (7 cases/28 passengers) and lower for people in non-window seats (4/83). The 7 window passengers never left their seat too. This factor makes other measures (masking) of prevention important.
Masks are Beneficial
Regardless of seating, evidence shows masks help to reduce transmission as shown by descriptive and modeling studies assessing the impact of masks on planes. Two public health reports assessed the transmission rates in the presence of rigid masking. The results affirmed low transmission with masking:
- The first flight study had 25 index cases and 2 secondary cases. One person was sitting next to a row with 5 index cases.
- On 5 Emirates Airlines with food service and passengers totaling more than 1500, no secondary cases were identified despite 58 index cases.
A 2021 modeling study was published with a few interesting findings.
- On a 2-hour flight with no masks, the average probability of infection was 2%. But if one sat next to an index case, the probability rose to 60%.
- On a 12-hour flight with no masks, the average probability of infection is 10% (or 1 in 10). If one sat next to an index case, the probability rose to 99%.
Besides wearing masks on planes, probabilities can vary with the wearing of high or low efficiency masks. Rick increases if not worn 100% of the time. The removal for meals increases risk. Passenger count and proximity to one another matters. The more the space, the less the risk.
Transmission on a plane impacts those on the plane. The transported infections can spill over and drive community transmission, too. An international flight landed in Ireland in the summer of 2020. Despite low occupancy on the plane, 13 secondary cases occurred equating to a 9.8-17.8% attack rate. Outward transmission resulted in a spread to 59 cases in six of eight health regions in Ireland requiring national oversight.
The Bottom line or Reality
Planes have excellent filtration/ventilation systems. Vaccines are highly effective. However, no mitigation measure is perfect. Wear your mask while traveling, especially with increasing case trends. Mask wearing is not that big of an inconvenience for good health. There may be other health consequences resulting from contracting Covid. Consequences not widely known. Another topic for later.